1. Technical Field
The present invention generally relates to the separation of gases. More specifically, the present invention relates to separation of light and heavy components of a feed gas via centrifugation. Even more specifically, in embodiments, the present invention relates to gas separation via centrifugation utilizing a porous rotor.
2. Background of the Invention
Separation of gases, especially gases having similar molecular weights, is a challenging task. Isotope separation is a particularly difficult and energy intensive activity. For example, separating uranium into naturally-occurring uranium-238 from uranium-235. Around 99.284% of naturally-occurring uranium is uranium-238, while about 0.711% is uranium-235, and about 0.0058% is uranium-234. Enriching uranium is difficult because the two isotopes have very nearly identical chemical properties, and are very similar in weight: uranium-235 is only 1.26% lighter than uranium-238. Separation methods, including diffusion techniques and centrifugation, that exploit the slight differences in atomic weights of isotopes have been employed to separate isotopes of uranium and produce depleted uranium consisting mainly of the 238 isotope, and enriched uranium having a higher-than-natural quantity of the uranium-235 isotope.
Diffusion techniques are used to separate similar-molecular weight gases. Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes, typically silver-zinc membranes, and separating the different isotopes by difference in diffusion rates. This produces a slight separation between the molecules containing uranium-235 and uranium-238, as uranium-238 is heavier and thus diffuses a bit more slowly than uranium-235. Thermal diffusion applies the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter uranium-235 gas molecules will diffuse toward a hot surface, and heavier uranium-238 gas molecules will diffuse toward a cold surface.
Centrifugation is also known for the separation of gases of similar molecular weight. A gas centrifuge is a separating machine specifically developed to separate uranium-235 from uranium-238 by applying forces to the gas mixture by placing material inside a mechanism that rotates the material at high speed. The gas centrifugation process uses a large number of rotating cylinders in series and parallel formations. The rotation of each cylinder creates a strong reactive centrifugal force accelerating molecules based upon mass and causing heavier gas molecules containing uranium-238 to move toward the outside of the cylinder and the lighter gas molecules rich in uranium-235 to collect closer to the center. Gas centrifugation requires much less energy to achieve similar separation to older gaseous diffusion processes, and thus gas centrifugation has largely replaced gaseous diffusion as an enrichment method. The Zippe centrifuge is an enhancement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinder is heated, producing convection currents that move the uranium-235 up the cylinder, where it can be collected.
A feature common to all large-scale enrichment schemes is that they employ a number of identical stages to produce successively higher concentrations of uranium-235. Each stage concentrates the product of the previous step further before being sent to the next stage. Similarly, the tailings from each stage are returned to the previous stage for further processing. This sequential enriching system is called a cascade. For example, gas centrifugation is performed with multiple centrifugal runs using cascades of centrifuges.
Another expensive gas separation, for example, is separation of undesired components from methane obtained downhole. Removal of carbon dioxide and other components from the methane to provide a pipeline-grade methane generally involves the use of very low temperatures and/or amine units. Such units can be costly and their operation time-consuming.
Accordingly, in view of the art, there is a need for efficient and economical apparatus and methods of separating gases. Desirably, the separation is performed in the absence of a large cascade of separation stages and/or in the absence of cooling.